Field of the Invention
The present invention relates to an image display apparatus which displays a larger projection image by combining a plurality of projection images, a method of controlling the same, and a non-transitory computer-readable storage medium.
Description of the Related Art
Recently, there have been widely used apparatuses which provide various kinds of information by using large image display apparatuses regardless of whether they are used outdoors or indoors. For example, many large image display apparatuses which provide various types of advertisements and environmental images exist in front of train stations and commercial districts. These large image display apparatuses include liquid crystal displays, plasma displays, and projection type image display apparatus (to be referred to as image display apparatuses hereinafter). When a screen size is 100 inches or more or the screen is not flat, in particular, a large image display apparatus is formed by using a multi-projection system which displays an image by combining a plurality of image display apparatuses. This is because a multi-projection system can easily display a large screen by combining a plurality of image display apparatuses, and can display images on a screen even if it is not flat.
A multi-projection system projects adjacent projection images while partially overlapping them so as to make the boundaries between the adjacent projection images inconspicuous. In this case, since a plurality of images overlap each other, overlapped areas where projection images overlap each other have a higher luminance than the remaining areas. There is known a method of suppressing the luminance signals of projection images in overlapped areas so as to make overlapped images have the same luminance after overlapping as that in areas other than the overlapped areas.
This method, however, cannot prevent a phenomenon called a misadjusted black level, in which black looks gray instead of black. A display element is designed to generate a projection image from an input image signal. Owing to this characteristic, even if a luminance signal is 0, that is, an input image is black, the image projected by the image display apparatus is not a black color equivalent to that of black body radiation. Assume that when the image display apparatus projects an image with a luminance signal of 0, that is, a black image, the luminance of the projection image is represented by black level (BL). In this case, since the luminance signal of the black image is 0, it is not possible to further suppress the luminance signal. Therefore, in an overlapped area where projection images from n image display apparatuses overlap each other, even if black is displayed, the black area has a luminance of n×BL. That is, misadjusted black level becomes prominent.
In addition, in a reflective liquid crystal display element, radiant light from a light source is reflected by members in the element, and the reflected light leaks outside the range of a projection image. For this reason, when constructing a multi-projection system by using image display apparatuses to which reflective liquid crystal display elements are applied, this leaked light leaks into the projection image area projected by another image display apparatus. This raises a problem that the luminance of the peripheral area of the overlapped area increases due to the influence of this leaked light. In addition, since the luminance of the peripheral area of the overlapped area increases, the misadjusted black level in the peripheral area of the overlapped area becomes prominent.
In consideration of such problems, it is possible to reduce the value of BL by controlling the light source of the image display apparatus and suppressing the light beam emitted by the light source. This can eventually reduce the misadjusted black level in the overlapped area. Furthermore, suppressing a light beam in a peripheral portion of a display element can reduce the light reflected by members in a reflective liquid crystal display element, thereby eventually suppressing an increase in luminance in the peripheral area of the overlapped area. Conventionally, there have been proposed techniques of controlling the light source of an image display apparatus. The technique disclosed in Japanese Patent Laid-Open No. 2012-89454 is a technique of controlling the light beams emitted from a solid-state light source array including a plurality of solid-state light sources at the duty ratio between an emission period and a non-emission period. In addition, Japanese Patent Laid-Open No. 2013-114980 discloses a technique concerning a light source apparatus designed to generate fluorescence by using excitation light emitted from two light sources. Japanese Patent Laid-Open No. 2013-76837 discloses a technique concerning a light source apparatus having a mechanism having a plurality of lenses fixed on the optical axis of light emitted from a light source. Japanese Patent Laid-Open No. 6-18806 discloses a technique of forming a light beam.
The technique disclosed in Japanese Patent Laid-Open No. 2012-89454 has a problem that if there is no emission period/non-emission period synchronization between image display apparatuses, projection images suffer from flickering and contrast inconsistency.